1. Field of the Invention
This invention relates to a faceplate arrangement for cathode ray tubes, and in particular to a faceplate arrangement including optical filter means for improving the quality of images produced by monochromatic cathode ray tubes having powdered-layer luminescent screens.
2. Description of the Prior Art
Conventional cathode ray tube display systems use either a single display tube or three monochromatic projection tubes. The display tube produces a viewable image at its faceplate. The projection tubes simultaneously project respective red, green and blue images onto a projection screen where they collectively form a viewable polychromatic image. In either type of system, the image contrast is adversely affected by halo. This is an undesirable ring or series of rings surrounding the luminescent image of the tube's scanning electron beam. In projection tube systems the image brightness is also substantially dependent on the angle at which light rays are emitted from the tubes' luminescent screens.
The causes of halo and the angle dependence of image brightness can be seen by referring to FIG. 1, which illustrates a typical prior art faceplate arrangement 10 of a projection tube spaced from a focusing lens 12, both shown in cross-section. The lens 12 magnifies the image formed by light rays received from the faceplate arrangement 10, and projects the image onto a relatively large reflective or transmissive projection screen (not shown).
The arrangement 10 includes a glass faceplate 14, a powdered luminescent screen 16 deposited onto the faceplate, and a reflective layer 18 of an electrically-conductive material such as aluminum. Typical thicknesses for the faceplate, the luminescent screen, and the reflective layer are 10 millimeters, 50 microns and 0.1 microns, respectively. The layer 18 is provided to collect excited electrons from the screen and to reflect toward the faceplate backwardly-directed light rays passing through the screen, thereby increasing the amount of light reaching the lens 12. As is subsequently explained, however, the layer 18 not only increases the amount of useful light reaching the lens 12, but also increases light contributing to halo.
Although FIG. 1 is not drawn to scale, it demonstrates how light rays emitted by the luminescent screen are transmitted through the faceplate arrangement 10. When an electron beam 20 excites the luminescent screen 16 at a spot such as that centered on point 22, a multiplicity of light rays are emitted at different angles. All angles are measured relative to a line 24 originating at the point 22 and passing perpendicularly through a faceplate-screen interface 26 and a faceplate-air interface 28.
All light rays emitted toward the interface 26 are at least partly reflected back into the powdered luminescent screen 16, as rays I.sub.B. These rays I.sub.B will be scattered in all directions by the particles in the luminescent screen. Those rays scattered toward the reflective layer 18 will be redirected toward the faceplate 14. The lateral shift between point 22 and any point at which one of these scattered rays is incident at the interface 26 is on the order of the thickness of the luminescent screen itself (e.g. 50 microns) and thus does not substantially increase the diameter of the luminescent electron beam spot, which is typically about 500 microns.
The light rays emitted from point 22 which pass through the faceplate-screen interface 26 reach the faceplate-air interface 28 at the front of the tube. Portions I.sub.L of these rays, emitted from point 22 at angles between 0.degree. and .theta..sub.COL, pass through interface 28 and are collected by lens 12, ultimately forming the image on the projection system screen. Portions I.sub.M emitted from point 22 at angles greater than .theta..sub.COL totally miss the lens, causing decreased image brightness. At least a portion I.sub.R of each ray reaching the interface 28 is reflected back toward the interface 26. The reflected rays I.sub.R reaching the interface 26 are laterally shifted from the point of origin 22 by distances on the order of the faceplate thickness (e.g. 10 millimeters). These laterally-shifted rays are back-scattered by the combination of the powdered luminescent material 16 and the reflective layer 18 and form a number of concentric ring-shaped halos around the image of the electron beam spot, causing decreased image contrast.
U.S. Pat. No. 4,310,784 to Anthon et al. discloses a cathode ray tube faceplate construction for suppressing halo. The construction consists of a clear glass faceplate having an anti-reflection coating on its outside surface and an angle-sensitive thin film interference coating between its inside surface and a phosphor screen. The outside anti-reflection coating is provided to reduce reflection back into the faceplate of luminescent light rays incident thereto at small angles, thereby suppressing a central portion of halo surrounding a luminescent spot. The inside interference coating is provided to reflect luminescent light rays incident thereto at large angles, thereby suppressing a ring-like outer portion of the halo surrounding the central portion.
In principle, an inside interference coating should, by itself, effectively decrease halo and increase brightness. Such a coating should reflect back into the powdered luminescent screen all rays which would otherwise contribute to halo. Some of these reflected rays would be redirected toward the faceplate, increasing image brightness. The coating should operate as a filter having a sharp cutoff, reflecting all rays which significantly contribute to halo (i.e. rays emitted at angles larger than a predetermined angle .theta..sub.H), and passing all rays emitted at smaller angles. This angle .theta..sub.H would be selected to meet the design criteria for the particular system in which the tube is used. For example, in a display tube system the angle .theta..sub.H could be made equal to the minimum angle of emitted rays which contribute to the innermost halo ring (typically 45.degree.). In a projection tube system the angle .theta..sub.H could be made equal to the angle .theta..sub.COL (typically 25.degree.-30.degree.). Anthon's interference coating, however, produces a gradual increase in reflectivity with angle, reaching only about 60% reflectivity for rays incident at 45.degree. while producing substantial reflectivity even for rays incident at angles between 0.degree. and 30.degree..